For sodium vapor high-pressure lamps it is known that, by means of a cermet starting line which is situated on the surface of the PCA ceramic and which is connected to an electrode, and is also designated as a hybrid antenna, the starting voltage can be decreased by comparison with the known systems and, given the same starting voltage, the luminous efficiency can be increased by increasing the xenon pressure; in this respect, see WO 2010/004472.
In WO 2010/004472, a distinction is made between an active and a passive hybrid antenna. The passive hybrid antenna is substantially based on a capacitive coupling of an electrode to the hybrid antenna. In order to achieve an optimum effect, the impedance between hybrid antenna and electrode is intended to be less than 10 kΩ. If a starting unit having an operating frequency of 300 kHz is used, then a coupling capacitance of approximately 55 pF is required in order to realize this condition. Said coupling capacitance can be achieved if, in the case of the sodium vapor high-pressure lamp having a leadthrough diameter of 3 mm and a distance between leadthrough and hybrid antenna of 50 μm, the hybrid antenna is embodied in the form of a cylinder having a height of more than 4 mm, which cannot be realized in practice.
For practical reasons, therefore, an active antenna is advantageous in which the hybrid antenna is connected to the electrode directly or via a connection having a certain ohmic resistance. WO 2010/004472 proposes realizing an electrically conductive connection or a connection having a certain contact resistance, which should not exceed 10 kΩ, but is preferably approximately 10 to 200Ω. For this purpose, it is possible to deposit an electrically conductive layer onto the glass solder by known methods, with the result that the hybrid antenna is electrically connected to the leadthrough of an electrode. What is disadvantageous is that the metals that can be deposited with a sufficiently high melting point and a coefficient of thermal expansion similar to that of the glass solder are not compatible with the existing manufacturing techniques for high-pressure discharge lamps and the integration of new production installations into the existing production processes is therefore required.
WO 2010/004472 also proposes using a conductive glass solder. The latter could be produced by adding a metal, e.g. tungsten, molybdenum, niobium, to the known glass solder powder. This new glass solder has to have a coefficient of thermal expansion similar to that of the known insulating glass solder, it has to produce a good connection to the PCA ceramic and the leadthrough, e.g. composed of niobium, and it has to have sufficient high resistance to diffusion of sodium at the high operating temperatures present of approximately 730° C. What is disadvantageous is that the development and the testing of such a conductive glass solder are very complex.